1. Field of the Invention
This invention relates to an implanted auditory prosthesis. The invention relates particularly, but not necessarily exclusively, to a method of, and a control system for, charging of a battery of a totally implantable auditory prosthesis such as a cochlear implant or an implanted hearing aid.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. In some cases, a person may have hearing loss of both types. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss is often helped by use of conventional hearing aids which amplify sound so that acoustic information reaches the cochlea and the hair cells.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence of, or destruction of, the hair cells in the cochlea, which convert acoustic signals into nerve impulses. These people are thus unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to, or absence of, the mechanism for nerve impulses to be generated from sound in the normal manner.
It is for this purpose that cochlear implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve.
Typically, cochlear implant systems consist essentially of two components, an external component, commonly referred to as a processor unit and an internal, implanted component, commonly referred to as a stimulator/receiver unit, the latter receiving signals from the processor unit to provide the sound sensation to a user.
The external component includes a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts speech into a coded signal, a power source, for example a battery, and an external transmitter antenna coil.
The coded signal output by the sound processor is transmitted transcutaneously to the implanted stimulator/receiver unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter antenna coil which is positioned to communicate with an implanted receiver antenna coil of the stimulator/receiver unit. Therefore, the communication serves two essential purposes; firstly to transmit, transcutaneously, the coded signal and, secondly, to provide power to the implanted stimulator/receiver unit. The transcutaneous link is, normally, in the form of an RF link, but other links have been proposed and implemented with varying degrees of success.
The implanted stimulator/receiver unit includes, in addition to the receiver antenna coil that receives the coded signal and possibly power from the external processor component, a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlear electrode assembly which applies the electrical stimulation via the basilar membrane to the auditory nerve producing a hearing sensation corresponding to the originally detected sound.
Recently, the Applicant has developed a totally implantable cochlear implant where all the components, including the microphone, are implanted subcutaneously. This results in a more versatile system providing the recipient with greater freedom and ability to use the implant in what would previously have been regarded as adverse environments, eg. wet environments. The Applicant's implant is described in greater detail in PCT/AU01/00769 which is incorporated herein by reference. The implant is powered by an implantable rechargeable battery which receives charging signals, when required, transcutaneously via an external charging device and an implanted receiver antenna coil. Because the battery is part of an implanted system, there is a need to make the battery life as long as possible to reduce the frequency of explantation and/or re-implantation of the implant for the purposes of battery replacement.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.